1. Field of the Invention
This invention generally relates to a method and an apparatus for mounting soldering balls onto corresponding electrodes on an upper surface of a workpiece, such as a substrate or a comparable electronic component, to form soldering bumps thereon.
2. Prior Art
In the field of fine electronic component mounting technologies, a recently developed technology is the utilization of soldering balls for forming terminals or connecting the same on a substrate or a comparable electronic component. One typical method for shifting and mounting such soldering balls onto electrodes of a substrate will include steps of providing a plurality of absorbing holes on an absorber head as much as the electrodes of a workpiece to be processed, absorbing soldering balls from a ball reservoir at the absorbing holes of the absorber head, and transporting and mounting the absorbed soldering balls onto the workpiece by shifting the absorber head.
FIG. 14 is a side view showing a typical conventional mounting apparatus for mounting soldering balls onto electrodes of a workpiece. Reference numeral 1 represents numerous soldering balls 1 stored in a ball reservoir 2 as material for forming bumps. Reference numeral 3 represents a pickup head which is moved in an up-and-down direction by a dedicated elevation unit (not shown).
The pickup head 3 has an inside hollow space covered by a lower surface having a plurality of absorbing holes. Each absorbing hole has a circular opening whose diameter is smaller than the diameter of soldering ball 1. To absorb soldering balls 1 at these absorbing holes, vacuum is introduced into the inside hollow space of pickup head 3.
After soldering balls 1 are absorbed at the lower surface, the pickup head 3 is lifted upward by the elevation unit and then shifted in a horizontal direction by a shift table (not shown) until the pickup head 3 is positioned just above a workpiece, such as a substrate 4, which is fixedly held by a damper 6.
Subsequently, the pickup head 3 is lowered until soldering balls 1 arrive at corresponding electrodes 5 of substrate 4 and then supply of vacuum is stopped to release soldering balls 1 onto corresponding electrodes 5, thereby accomplishing a simultaneous, batch mounting operation.
The layout of numerous holes on the lower surface of pickup head 3 is determined based on the correspondence to the pattern of electrodes 5, it is thus fundamentally necessary to surely pick up soldering balls 1 at all of these absorbing holes and also to surely release all of conveyed soldering balls 1 on the corresponding electrodes 5 without failure.
However, according to the above-described conventional method, there is the problem that some of soldering balls 1 may be accidentally left in the holes of the pickup head 3.
More specifically, some of soldering balls 1 may be undesirably trapped so deeply in the absorbing holes that these soldering balls 1 are no longer disengaged from the absorbing head 3 because each soldering ball 1 is too soft to prevent deformation when it is forcibly sucked into the absorbing hole under a given vacuum pressure.
Failure of mounting or transferring all of soldering balls from pickup head 3 onto the substrate 4 will result in an undesirable production of defective substrate 4 which has one or more electrodes 5 missing soldering balls 1 to be mounted thereon.
These soldering balls 1, after being mounted on electrodes 5 of substrate 4, are heated up and melted down to form a bump on each electrode. In general, the formation of bumps on electrodes 5 is realized by a series of steps including a coating step of applying flux on electrodes or others and a bump forming step of heating, melting and hardening soldering bolls 1. However, even the most advanced mounting technology has not yet reached the level of automatically performing all of these steps as a sequential operation.